Release 0.9.4 "Face, meet palms" - Pixmaps, bug fixes

This release fixes embarassing numerical bugs introduced in 0.9.3 and
introduces a new `pixmap` type intended to capture screen pixel maps
including in color.  Builds for hardware calculators are also smaller
due to switching to a `nano` configuration for the compiler.

New features
============

* graphics: Add `pixmap` type

Bug fixes
=========

* Adjust unit conversion in `StdRnd`
* Fix parsing of `ΣY²` in equations
* Render non-fancy large exponents correctly (e.g. `1E123456789012345`)
* The precision of numerical constants now adjusts after changing `PREC`
* Avoid unnecessary purging of constants cache due to internal precision changes
* Return tests values to rounded valus as validated with Wolfram Alpha
* directory: Fix `STOVX` and `Store` to non-current directories

Improvements
============

* Fix `sqrt` precision, notably for perfect squares (ouch)
* Improve support for recent ARM compilers
* Eliminate new `clang` warnings
* Eliminate warnings about function pointers in latest XCode
* Add documentation for `DisplayModesMenu`
* Precision adjustments are now more consistent across decimal functions